Te-rich CdTe surface by pulsed UV laser treatment for ohmic back contact formation

Pulsed UV laser treatments have recently been applied to polycrystalline CdTe solar cells to create an ohmic back contact. In this work, we investigate the surface stoichiometry variations produced by pulsed laser excitation using X-ray photoelectron spectroscopy (XPS). These results reveal surfaces that are becoming more Cd-rich, counter to what is expected from calculations and what has been measured by others. To understand this apparent disagreement, we have constructed a geometric model of the sample surface considering surface topography and a segregation of phases at the surface. From this, we infer that the laser treatments do produce a Te enrichment as expected, but that this enrichment is localized near the grain boundaries. Furthermore, the apparent Cd-enrichment occurs because the Te-rich region's contribution to the XPS intensity is lessened by topographic and surface sensitivity effects as well as a sub-monolayer of Cd found elsewhere on the film surface.